Various attempts have been made to remotely measure atmospheric variables used for weather prediction (e.g. temperature, humidity, and pressure) in inaccessible areas, such as over the Earth's oceans, using an aircraft. Previously developed systems make use of occultation and scattering of Global Navigation Satellite Systems (GNSS), for example signals using GPS, Glonass, and Europe's planned Galileo. The signals are used to calculate atmospheric water vapor content, temperature profiles, and ocean wave height and direction. The GNSS occultation method, while viable, in some instances may be limited in precision by the weakness of typical GNSS signals. GNSS occultation may also be limited in coverage by the relatively small number of GNSS satellites and their slow apparent motion across the sky.
One particularly important issue that must be dealt with when using GNSS satellites for weather predicting purposes is the sparse coverage afforded by GNSS occultation. Occultation measurements generally require that a GNSS satellite appear within a few angular degrees of the observer's horizon. If the satellite is too low, it may be occluded by the Earth. If it is too high, the signal's path through the atmosphere may not traverse the troposphere on its way to an aircraft flying at a cruise altitude. This can make the data obtained nearly useless for weather prediction.
The times when a GNSS satellite is near the Earth's horizon for any given aircraft are quite infrequent, and typically once an hour or so. Given that a jet aircraft typically covers about 1000 kilometers in an hour when travelling at a cruise altitude, the distance between occultation measurements is so large that they may give relatively little value for weather models.
A second important aspect is the relative weakness of GNSS signals. GNSS satellites are power-limited, so the strength of their signals is designed to be just barely adequate for a receiver to detect and track them under normal operating conditions. For navigation, “normal operating conditions” means the satellite is relatively high above the horizon and the receiver has up to a minute to achieve good synchronization to the signal. Thus, the signal is just strong enough to reach a typical mobile receiver after traversing a few miles of atmosphere. For meteorological measurements, however, the satellite should be near the horizon, so the signal would typically traverse hundreds of miles of atmosphere before arriving at the mobile receiver. In addition, for cases where the satellite is rising rather than setting, the receiver must synchronize to the signal in less than about a minute so that the receiver can make occultation measurements while the satellite is close to the horizon. With the low power signals of GNSSs, this requires large antennas and expensive receivers to be employed on an aircraft. These limitations would not be desirable for a weather predicting system that will be deployed using dozens, or possibly hundreds, of aircraft.
Still other ways of gathering water vapor data over the oceans, however, have all been subject to various limitations. Radiosondes may be sent out over the ocean, but these can be expensive to gather the frequency of data required. Currently, the National Weather Service (NWS) obtains information on the water vapor distribution from satellite information and from twice daily radiosonde launches at 93 sites around the continental U.S. (Coster, et al) The radiosonde network is expensive to operate. In addition to the expense, the balloons carrying the sonde packages take about an hour to reach the tropopause. Therefore, the atmospheric data obtained is not available for some time. Because there are not many radiosonde balloons available, the horizontal spatial density is too low, and time between launches too high, to observe rapid changes of the water vapor with time and position.
Marine vessels with suitable instrumentation may also be used in an attempt to collect atmospheric information. However, this method does not provide sufficient frequency of data and the vessels can be expensive to operate.
There presently is a NASA-funded program called “TAMDAR” that uses in-situ temperature, wind, and humidity measurements obtained by aircraft. This significantly improves weather forecasts over land areas where aircraft frequently ascend and descend through the troposphere. However, it has essentially no value over oceans. Over oceans the aircraft operate at cruise altitude, and thus well above the tropospheric phenomena that influence weather.
Satellite measurements over the oceans may reveal cloud formations and some limited data about temperature and humidity, but typically lack the vertical resolution needed for good forecasts.
Networks of GNSS receivers on land are typically unable to gather data for most of the Earth's surface, i.e. over the oceans. Poor tropospheric coverage over the oceans can lead to unreliable weather forecasts for the western United States, Western Europe, Australia, and occasionally Japan. Remote measurements of the troposphere using GNSS occultation from aircraft could improve this situation, but these measurements would still suffer from limited coverage and poor signal strength as discussed above.
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.